Planar optical waveguides are required for optical interconnection in integrated optical and optoelectronic devices. The methods used to manufacture such waveguides must be compatible with semiconductor processing methods used to manufacture other parts of the integrated devices.
Planar optical waveguides have been made by depositing a photosensitive monomer on a substrate and selectively exposing the deposited monomer to ultraviolet (UV) radiation. The UV radiation polymerizes the exposed monomer to provide polymer regions having a relatively high refractive index bounded by monomer regions having a relatively low refractive index. A further monomer layer is generally deposited over the partially polymerized layer for protection against surface flaws and contaminants which could couple light out of the polymerized regions. Unfortunately, the waveguides made by this method are unstable at the high temperatures which are used in some semiconductor processing methods. Consequently, all high temperature processing steps must be completed before the waveguides are defined. Moreover, this method generally requires two or more deposition steps.
Planar optical waveguides have also been made by depositing or growing a first layer of SiO.sub.2 on a substrate, depositing a layer of Si.sub.3 N.sub.4 on the first layer of SiO.sub.2, depositing a second layer of SiO.sub.2 on the Si.sub.3 N.sub.4 layer, and selectively removing a partial thickness of the second SiO.sub.2 layer in selected regions to lower the effective refractive index of the underlying Si.sub.3 N.sub.4 layer in those regions. This method requires three deposition or growth steps and one etch back step, all of which must be carefully controlled for satisfactory results.
Silicon-based planar optical waveguides have also been made by depositing or growing a first layer of undoped SiO.sub.2 on a substrate, depositing P-doped SiO.sub.2 on the layer of undoped SiO.sub.2, selectively removing regions of the P-doped SiO.sub.2 layer to expose regions of the first layer of undoped SiO.sub.2, and depositing a second layer of undoped SiO.sub.2 on the exposed regions of the first layer of undoped SiO.sub.2 and on the remaining regions of P-doped SiO.sub.2. The regions of P-doped SiO.sub.2 have a higher refractive index than the surrounding regions of undoped SiO.sub.2. This method also requires three deposition or growth steps and one etch back step, all of which must be carefully controlled for satisfactory results.
In U.S. Pat. No. 4,585,299, Robert J. Strain discloses a method for making silica-based planar optical waveguides in which boron, phosphorus, arsenic or germanium is implanted into a silicon substrate through a first mask and the substrate is oxidized through a second mask to provide a patterned SiO.sub.2 layer which incorporates the implanted dopant. The implanted dopant raises the refractive index of a central region of the SiO.sub.2 layer to provide a waveguide. This patent suggests that migration of the dopant during the oxide growth may be a problem.
Silicon-based planar optical waveguides have also been made by depositing or growing a layer of SiO.sub.2 on a substrate and selectively bombarding the SiO.sub.2 with H or B ions to define regions having a relatively high refractive index bounded by regions having a relatively low refractive index. The implantation process causes localized compaction of the SiO.sub.2 which locally increases the refractive index of the Si).sub.2. The presence of the implanted H or B ions may also modify the refractive index of the implanted SiO.sub.2. Unfortunately, the SiO.sub.2 is decompacted and the implanted H or B ions are redistributed by diffusion in the SiO.sub.2 layer if the waveguides are subjected to subsequent high temperature processing steps. The decompaction of the SiO.sub.2 and the migration of the implanted H or B ions degrades the refractive index profile defined by the implantation process. Consequently, all high temperature processing steps must be completed before the waveguides are defined.